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Woolly Oak Leaf Gall

11-28-14  woolly oak leaf gall IMG_3574Of the 2,000 kinds of galls found on North American plants, 800 different kinds form on oaks. One of these is the woolly oak leaf gall, produced by a tiny Cynipid wasp, Callirhytis lanata. This gall is usually attached to the mid-vein on the underside of an oak leaf, and looks like a ball of wool. It may be as large as three-fourths of an inch and is often bright pink or yellow in color, fading to light brown in the fall. Oak trees have lots of tannic acid in them (a defense which makes the tree unpalatable to herbivores), with the highest concentration found in oak galls. (The bitter taste is where the name “gall” originated.) It’s possible, since tannins are somewhat anti-microbial, high-tannin galls such as the woolly oak leaf gall may protect the larva against fungi and bacteria.

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10 responses

  1. Marv Elliott

    The tannin in oak is also well known by brewers. Oak staves in barrels are used to age whiskey. For the suds and science folks.

    December 3, 2014 at 1:40 pm

  2. Cliff Fairweather

    An excellent treatment of the evolutionary pressures driving gall morphology is available at

    December 3, 2014 at 2:50 pm

    • Thank you so much. It is, indeed, excellent!

      December 3, 2014 at 3:49 pm

  3. Penny March

    Did the wasp sever the vein–hence the death of the leaf beyond the gall?

    December 3, 2014 at 2:57 pm

    • You ask the best questions, Penny! No, I believe that is just coincidental, as you find them on green leaves in the summer. The wasp does not sever the vein that I know of, though I suppose it’s possible that that might happen when it exits and chews its way out of the gall?

      December 3, 2014 at 3:44 pm

  4. Some of my students and I found hundreds of these on the ground in the oak woods around Walden Pond in the autumn, 2013. This year we didn’t find a single one anywhere in Concord, which made us wonder about their cycles and what might effect them.

    December 3, 2014 at 3:41 pm

    • Fascinating. I’m sure there’s a whole series of events that is responsible for the lack of this particular gall, but I’m afraid I don’t know the specifics!

      December 3, 2014 at 3:47 pm

  5. Irma Graf

    I always learn from you, Mary! Never knew where the term “gall” came from. Now it makes sense.

    December 3, 2014 at 10:41 pm

  6. Fascinating – about 40% of galls are on oaks. I love the antimicrobial bit. It would explain why they are so slow to decompose.

    December 3, 2014 at 11:49 pm

    • I hadn’t thought about the decomposing part…how insightful of you, Eliza.

      December 4, 2014 at 1:05 am

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